The behavior of such a channel has been modeled mathematically by a delay line with a continuum of taps each of which acts as if the signal underwent uncorrelated scattering and multiplicative fading, the outputs of which taps are then all appropriately combined by summation. If the fading is wide sense stationary, such channels are sometimes referred to in the art as WSSUS channels. More generally, such channels can also be mathematically modeled by a bank of filters. Under certain propagation conditions which generate very extensive dispersion, a conventional method of providing equipment which simulates the behavior of such channels normally requires a very large number of uncorrelated taps so that the cost of producing such a channel simulator becomes relatively costly and in some cases prohibitively expensive. Moreover, under other circumstances the accuracy achieved by the use of such conventional channel simulation techniques is simply not adequate.
Channel simulators of such type have been described, for example, in the following publications:
1. P. A. Bello and L. Ehrman, "Troposcatter Model Performance Prediction with a Complex Gaussian Troposcatter Channel Simulator", Conf. Rec., IEEE Int. Conf. Commun., 1969, pp. 48-11 to 48-16.
2. L. Ehrman, "Tropo diversity simulator development", SIGNATRON, INC., USAF Rome Air Development Center, Final Report Contract F30602-73-C-0216, July 1974.
3. J. J. Bussgang, E. H. Getchell, B. Goldberg and P. F. Mahoney, "Stored Channel Simulation of Tactical VHF Radio Links", IEEEE Trans. Communications, Vol. COM-24, pp. 154-169, February 1976.
Such fading channel simulators have been used to test high frequency and troposcatter systems and utilize tapped delay lines, one for each diversity branch, each of which uses a plurality of uncorrelated tap weights. Simulators of the type described above can demonstrate radio frequency performance normally only for special types of random channels where the signal scattering vs. delay characteristic occurs at discrete times. Such simulation may be adequate for comparing different radio frequency channel operations and for verifying such operations. However, it is normally not adequate for determining whether a radio frequency communication system, for example, will operate within selected specifications on a given communications media link. Moreover, in such simulators the actual power impulse response function of the media link cannot be prescribed by the user of the simulator. Instead existing simulators require that the user specify the power level of each uncorrelated tap weight and further require the user to determine what those power levels should be from the given power impulse response.
All such currently known high frequency simulators operate with uncorrelated tap weights, simulators for troposcatter systems normally being implemented with analog circuits, while those for other high frequency simulators normally being implemented digitally by using an array processor, for example.
It is desirable to provide channel simulators, however, which, in a more cost effective manner than previously proposed simulators, will provide accurate modeling by using, for example, fewer than the conventional number of taps on the delay lines involved. Such a simulator further should be designed to act on the signal transmitted by an actual radio frequency system in order to simulate signals that would be received in one or more diversity receivers of such a system. Such a simulator should be usable to test the performance of radio frequency systems and to compare such performance with theoretical performance characteristics using simulator settings for either the parameters of a hypothetical fading channel or for the measured characteristics of an actual channel.